1. Field of the Invention
This invention relates to fast-setting magnesium phosphate cements. More particularly, it relates to an improved process for the preparation of a particulate solid activator comprising a porous material containing absorbed ammonium phosphate which, when admixed with MgO, reacts in the presence of an aqueous component at ambient temperatures to form a magnesium phosphate fast-setting cement.
2. Related Developments
Cementitious compositions which set in a short period of time and prepared by mixing a magnesium-containing component with a P.sub.2 O.sub.5 material and an optional aggregate are popular for use in repairing or patching cracks, holes and spalled areas of highways, airfield pavements and bridge decks, as well as for a variety of commercial and industrial applications.
U.S. Pat. No. 3,202,520 (Enoch, Aug. 8, 1965) discloses a rapid set non-hygroscopic cement composition consisting essentially of a blend of a phosphate derived from phosphoric acid and of alumina, and containing from 15 to 55 weight percent of P.sub.2 O.sub.5, from 45 to 75 weight percent of alumina, and from 5 to 18 weight percent of magnesium oxide. The phosphoric acid and alumina are blended at room temperature, with the alumina added to the liquid phosphoric acid. The resultant mixture is dried above 100.degree. C. for a sufficient time to remove all moisture. The resulting dried material is milled to pass through a 325 mesh screen prior to being dry blended with the MgO.
U.S. Pat. No. 3,525,632 (Enoch, Aug. 25, 1970) discloses a rapid setting concrete cement composition which comprises dry blending magnesium-containing compounds, aluminum-containing compounds and phosphorus-containing compounds from stated sources, in stated weight percentages, and then pulverizing the blended material without a separate drying stage. The phosphorus-containing compound is a mixture derived from phosphoric acid, and at least 50 weight percent of one of trimagnesium phosphate, aluminum orthophosphate, and phosphoric anhydride. This reference states that the compositions of U.S. Pat. No. 3,202,520 (discussed above), which require a drying step to remove moisture, resulted in an increased cost. This drying step is carried out in expensive stainless steel equipment to avoid corrosion and contamination problems. Further, during drying, there is a pronounced tendency for the mixture to cake within the dryer causing serious production problems.
U.S. Pat. No. 3,879,209 (Limes et al., Apr. 22, 1975) discloses a process for a fast-setting concrete comprising establishing a mixture for an aggregate, containing at least 10 percent by weight of magnesia, and ammonium phosphates in aqueous solution. This patent discloses that a commercial agricultural fertilizer product known as sequestered phosphatic solution (SPS) is particularly suitable as a source of the ammonium phosphate. No separate drying of the SPS is disclosed in this reference.
U.S. Pat. No. 4,059,455 (Limes et al., Nov. 22, 1977) discloses a process for making a fast-setting concrete comprising establishing a mixture of an aggregate, containing at least 1 percent magnesia, and ammonium phosphates in aqueous solution. No separate drying of the ammonium phosphate solution is disclosed in this reference.
U.S. Pat. No. 4,174,227 (Tomic, Nov. 13, 1979) discloses a grouting system comprising an acidic reactive component comprising at least one acidic oxyphosphorus compound selected from phosphoric acids, anhydrides of phosphoric acids and salts of phosphoric acids with multivalent metal cations, and a basic reactive component comprising at least one basic metal compound of a Group II or Group III metal capable of reacting with the oxy-phosphorous compound(s) in the presence of water to form a monolithic solid. This reference does not disclose a separate drying step for the acidic reactive component.
U.S. Pat. No. 3,821,006 (Schwartz, June 28, 1974) discloses a patching composition consisting essentially of a reactive component of MgO with an acid phosphate salt and an inert aggregate component. The particle size of the inert aggregate component is related to the compressive strength of the cement formed at an early cure age. This reference does not disclose a separate drying step for the acidic component.
The mixtures employed in the methods of the references discussed above, set and develop usable strength much more rapidly than conventional materials. Nonetheless, each of these references is limited, as discussed above, and therefore distinguishable from the process of the present invention in several ways.
U.S. Pat. No. 3,673,111 (Hovarth, June 27, 1972) discloses a process for manufacturing a solid phosphoric acid catalyst in which a siliceous adsorbent is admixed with a polyphosphoric acid. The mixture is heated at an elevated temperature and thereafter extruded. The extrudate is further treated by drying first in a steam atmosphere and then in a dry air atmosphere. This reference does not teach a process for producing solid P.sub.2 O.sub.5 material suitable for use in fast-setting cements but rather teaches a process for making a catalyst.
U.S. Pat. No. 3,475,188 (Woodhouse et al., Oct. 28, 1969) discloses a dry refractory composition consisting essentially of phosphates absorbed on diatomaceous earth, up to 80 percent by weight being absorbed thereon, magnesium oxide and refractory aggregate. Compositions and processes in which well above 80 percent by weight of phosphates absorbed on diatomaceous earth are present would be advantageous but are not contemplated by this reference. A separate drying step for the phosphates absorbed on diatomaceous earth is also not disclosed. This reference is directed to the refractory arts wherein curing at high temperatures of the phosphate/MgO blend is contemplated before substantial compressive strength develops. The invention, in contrast, is directed to the fast-setting magnesium phosphate cement art wherein the phosphate/MgO blend reacts at ambient temperatures in the presence of an aqueous component to yield substantial compressive strengths shortly after hydration.
U.S. patent application Ser. No. 788,664, filed Oct. 17, 1985, discloses a particularly effective solid phosphorus pentoxide material suitable for use in fast-setting magnesium phosphate cements. This material is prepared by a process which comprises mixing a porous material with a liquid phosphorus pentoxide material and heating the mixture until a dry solid is produced. This solid phosphorus pentoxide material can then be dry blended with a solid component comprising magnesium oxide, magnesium hydroxide, magnesium carbonate or mixtures thereof. An aggregate can optionally be added to this dry blend. When hydrated, this dry blend has been found to be particularly advantageous as a fast-setting magnesium phosphate cement. An aqueous solution of ammonium phosphate has been found to be particularly advantageous when used as the phosphorus pentoxide material that is mixed with the porous material. Heating this mixture of ammonium phosphate and the porous material to form a dry solid can yield inconsistent results, e.g., the drying time can be extremely long and/or the resulting dry solid can be difficult or even incapable of being milled to a preferred mesh size.
U.S. patent application Ser. No. 735,907, filed by Tieckelmann et al., May 20, 1985, discloses an improved process for drying ammonium phosphate solution absorbed onto porous material which comprises adding an effective amount of an acid, preferably orthophosphoric acid, to the solution prior to the drying step. The improvement comprises a substantial shortening of the drying time. The resulting dry activator can be used in fast-setting magnesium phosphate cements.
A process for improving this drying process while maintaining the advantages of these particular compositions of magnesium phosphate fast-setting cements would be advantageous.